What Drives the Inter-Annual Variability in the Advective Cooling of the Oregon Inner Shelf?

Temperature and velocity measurements from summertime observations in 15 meters of water over 2 decades are used to investigate the inter-annual temperature variability over the inner shelf off Oregon. A two-dimensional heat budget model is used to estimate the capacity of the cross-shore heat flux to buffer the net surface warming each summer. The net surface heat flux is a consistent summertime source of heat to the inner shelf from year-to-year, with a standard deviation that is less than 15% of the inter-annual mean. Despite surface heating the water temperatures do not warm up over the summer. For all of the years examined there is no trend in the summertime mean temperatures, and the inter-annual range in mean temperature (1.7ºC) is similar to the standard deviation in daily temperature (1.6°C). The mean cross-shore circulation advects relatively warmer water offshore near the surface and cooler water onshore at depth, acting to cool the inner shelf and buffer the surface warming. Most years (10 out of 14), the two-dimensional heat budget roughly closes with a residual less than 10% of the leading term. In the years when the heat budget does not balance an additional source of cooling is needed to close the budget. A comparison of the heat budget to the inter-annual variability in other fields such as alongshore wind stress, stratification, and alongshore currents does not result in any clear patterns, and we investigate the intra-seasonal dynamics in order to explain the results. Many coastal species are sensitive to changes in water temperature. A better understanding of the processes that contribute to warming or cooling of the coastal ocean will improve predictions of the impact year-to-year changes in the local winds and circulation have on coastal temperatures.